Plant pathology or phytopathology is the science of dealing with plant diseases and their control. Plant pathologists study plant diseases caused by fungi, bacteria, viruses, nematodes, and parasitic plants.

They also study plant disorders caused by nutrient imbalances, air pollution, and other unfavorable growing conditions.

History of Plant Diseases

Plant diseases have had profound effects on mankind through the centuries as evidenced by Biblical references to the blasting and mildew of plants.

The Greek philosopher Theophrastus (370-286 B.C.) was the first to describe maladies of trees, cereals, and legumes that we currently classify as leaf scorch, rots, scab, and cereal rust.

The Romans were also aware of rust diseases in their grain crops. They celebrated the holiday of Robigalia when sacrifices of reddish-colored dogs and cattle were made in an attempt to appease the rust god Robigo. With the invention of the microscope in the 17th century, fungi and bacteria associated with plants were investigated.

In 1665, Robert Hooke published the first illustration of rust on a rose leaf. Advances in the study of diseases were hampered by the widely held belief in the theory of spontaneous generation.

This theory, held by most people in the mid-18th century, considered pathogenic or disease-causing microorganisms as products of disease rather than causes of disease. Epidemics of late blight of potato devastated Ireland in 1845 and 1846.

These epidemics dramatized the effect of plant diseases on mankind. Tragically, these epidemics caused famine and death for over a million people. Between 1845 and 1860, death and migration accounted for the loss of nearly one-third of Ireland’s population.

In 1861, a German botanist, Anton De Bary, proved that a fungus (Phytophthora infestans) was the causal agent of the late blight of potatoes. This was a milestone in the study of plant diseases since it showed that a fungus was indeed the cause of a plant disease rather than an organism simply associated with the disease.

Two years later, Louis Pasteur proposed his germ theory of disease finally disproved the theory of spontaneous generation, and changed the way modern science investigated the diseases of all living organisms.

A few examples of plant disease epidemics that have resulted in devastating plant losses in the United States include: chestnut blight, introduced in 1904, virtually eliminated chestnut trees from North America; citrus canker, introduced in 1910, and a closely related bacterium called citrus bacterial spot discovered in 1984, resulted in the destruction of millions of citrus trees; white pine blister rust, introduced in 1912, caused large economic losses in the timber industry; and Dutch elm disease, introduced in 1930, continues to destroy large numbers of elm trees from the East Coast to the Pacific Northwest.

What is a Plant Disease?

There are many ways to define what a plant disease is. However, simply put, plant diseases involve profound changes within the host that cause a disruption of normal plant function.

A good working definition of a healthy plant is one that can carry out its physiological functions to the best of its genetic ability.

Diseases are among the most important factors that can significantly diminish growth and yield, or reduce the usefulness of a plant or plant product.

Healthy or normal plants develop and function to the maximum of their genetic potential.

However, when plants are adversely affected by continuous irritation by a disease-causing agent, which interferes with normal development and functioning, plants are considered to be diseased.

This broad definition excludes injury or damage such as mechanical injury (e.g., lawn mower or weed-eater injury to trees); deer, rodent, and bird damage; hail damage; and lightning injury.

In addition to the reduction in growth, yield, and economic or aesthetic value of a plant or plant product, diseases may lead to the death of the whole plant or the destruction of the entire crop under conditions favorable for the disease.

Diseases may interfere with the absorption and translocation of water and nutrients from the soil to the various parts of the plant, may reduce the photosynthetic efficiency of the plant parts, may interrupt the translocation of photosynthetic products through the plant, or may interfere with the reproduction and storage of food reserves in the plant.

Diseases in plants are caused by either living (biotic, parasitic, or infectious) agents called pathogens, or non-living (abiotic, nonparasitic, or non-infectious) environmental factors.

Plant diseases may also be grouped by the causal agent involved (fungal diseases, bacterial diseases, viral diseases, nematode diseases, etc.), the plant part affected (root diseases, seedling diseases, leaf diseases, stem diseases, flower diseases, fruit diseases, tuber diseases, etc.), or the types of symptoms (damping-off, wilts, leaf spots, cankers, blights, galls, root knots, mosaics, storage rots, etc.).

Symptoms of Plant Diseases

Symptoms are the visible reactions of a plant to a disease and may suggest a causal agent.

A sampling of disease symptoms might include wilting, necrosis, abnormal coloration, defoliation, fruit drop, abnormal cellular growth, or stunting of the infected plant.

However, it is important to remember that different disease agents can cause similar symptoms in the same host. An equally important point to remember is that insect feeding can also cause disease-like symptoms in plants.

Signs of plant Diseases

Signs are the visible parts of the pathogen or its products seen on the host that can be used to identify the pathogen.

Examples of common disease signs include the white coating of mycelium visible on powdery mildew-infected leaves, mushroom growth on a tree limb, droplets of bacterial ooze running down a fruit tree twig, nematode cysts on plant roots, or dark fungal fruiting bodies visible in leaf lesions.

Causal Agents of Plant Disease

A pathogen is any organism that can cause disease. Pathogens cause infectious diseases that can spread from an infected plant to a healthy plant. Pathogens that cause infectious diseases include bacteria, fungi, viruses, nematodes, and parasitic plants.

Plant disease can also be caused by non-infectious or non-living factors. Causes of disease by non-living factors include unfavorable growing conditions, mineral deficiencies, and air pollution. Pathogens that cannot be cultured apart from their host are classified as obligate parasites.

Pathogens that can be cultured apart from their hosts on artificial media are called non-obligate parasites. In general, obligate parasites only attack very specific host plants, whereas non-obligate parasites typically have a wider range of plants they can infect.

Some pathogens are restricted to a single plant species, while others infect a single plant genus. Still, others attack a large number of hosts from many plant genera. There are also several levels of parasitism that pathogens can have with their hosts.

When a pathogen is capable of infecting a plant, the plant is considered susceptible to that pathogen. If a pathogen cannot infect a plant, then the plant is considered immune to that pathogen.

Plants can vary in their response to pathogens from high resistance (very little disease development), to partial resistance (moderate disease development), or high susceptibility (severe disease development).

Pathogens can vary in their degree of virulence on a susceptible plant ranging from highly virulent (causing severe disease symptoms) to weakly virulent (causing less disease).

Inoculum and Pathogen Dissemination

Inoculum is any part of the pathogen that can cause infection. Examples of inoculum include fungal spores, bacterial cells, virus particles, or nematode eggs.

The inoculum that survives the winter and causes the original or primary infection in the spring is called the primary inoculum.

The secondary inoculum causes additional infections throughout the growing season. Inoculum is sometimes present at the site where a plant is grown and can also be introduced from an outside source.

Inoculum already presents at a plant site includes soil pathogens or pathogens that overwinter on perennial weeds. Introduced inoculum includes infected plant material such as infected seeds, wind-blown fungal spores, and inoculum transmitted by insects.

Inoculum can be disseminated passively by wind, rain, and man. Inoculum can also be disseminated actively by insects and nematodes or fungal zoospores swimming through the water in the soil toward plant roots.

Only a fraction of any pathogen’s inoculum will ever land on a susceptible host. The vast majority of inoculum lands on material that cannot be infected. Most pathogens produce a tremendous surplus of inoculum.

Pathogen Survival

Pathogens in temperate climates must have a way of overwintering when their host plants are dormant or absent. In perennial plants, pathogens can survive in infected plant parts such as roots, bulbs, stems, and bud scales.

Annual plants, however, die at the end of the growing season and pathogens must survive in insects, seeds, or as resistant spores.

Factors Affecting Plant Disease Occurrence

Diseases in plants are an exception rather than a rule. Three factors, called the disease triangle (Fig. 1), must coincide for a plant to become diseased: the host, the pathogen, and the environment. The interaction between these three factors with time determines the occurrence and severity of a disease.

For the disease to occur, the following conditions must be met:

The host plant must be of a susceptible species or cultivar at the right stage of development (susceptible host).

The pathogen must be of a virulent race or strain and must be present in sufficient numbers (inoculum potential). The presence of appropriate vectors or other agents of dispersal is also necessary.

The environmental (atmospheric and soil) conditions such as temperature, humidity, rainfall, wind, moisture, light, soil type, texture and pH, the density of planting, aeration, and nutritional status (mineral deficiency or excess) must be favorable for disease development.